In semiconductor processing, one of the more critical process steps is the interconnection of two conducting layers on different levels that are separated by an insulating layer, especially when one of the conductive layers is the top metal layer. Presently, an underlying conductor layer is covered with an interlevel oxide layer and then the contact or VIA formed therein to expose the surface of the underlying conductor layer at a selected area. A top conducting layer is then patterned and interconnected through the contact or VIA with the underlying conducting material. This underlying conducting material can comprise either a conductive layer of polysilicon or metal or even the silicon surface itself.
To realize a conductive interconnection between the two layers, it is important that the contact interface between the underlying metal or silicon presents a low resistance without altering the characteristics of the underlying material, especially when the material is silicon. Further, it is important that the resistance between the contact interface itself and the upper metal layer also presents a low resistance.
One disadvantage with past processing techniques is "voiding" of the upper metal layer on the vertical surfaces of the contact opening or VIA. This can result from a number of factors. One factor which is prominent in the industry is that when sputtering or physical vapor deposition techniques are utilized to deposit a metal layer, the coverage is not conformal. Since this is an anisotropic process, vertical surfaces in a contact opening or VIA have only a relatively thin layer of metal formed on the vertical walls with a thick "build up" along the upper edges of the contact opening or VIA. The voiding typically occurs along these vertical surfaces. This can be solved by utilizing chemical vapor deposition of a conductive material. However, chemical vapor deposition processes are normally not suited to the types of metal, etc., that are required for upper levels, such as for aluminum metalization processes.